Properly translated read says 'they are Jews, but they are not, they are the synagogue". In both case it contest the sincerity of some who say they are Jewish, but are really Jewish. Much like some Liberal Christians say they are Christian, but they are not, such as they saying certain things are allowed when in fact they condemned in Scriptures , to condone things unholy to a Christian or Follower of Jesus is not true repentance from condemned practices - unbelievers are there true form.
U.S. confirms ISIS computer expert killed in air strike (Reuters)
from the article: That most Jewish of books